Compound thin-film solar cell and process for producing the same

ABSTRACT

A method of fabricating a thin-film compound solar cell having an n-type buffer layer formed therein for providing a heterojunction with a p-type compound semiconductor light absorbing layer formed on a back electrode by applying a chemical bath deposition (CBD) process using an aqueous solution for dipping the light absorbing layer to deposit particles on the surface thereof. In this process, the temperature of the solution is controlled from low to high to increase sizes of the particles to be deposited on the light absorbing layer so as to form the buffer layer which possesses a high optical transmittance, tight adherence to the light absorbing layer and conformity with the transparent electrode formed thereon even if it would be made of InS material generally possessing a small bandgap and hard to pass light of short wavelengths.

BACKGROUND OF THE INVENTION

The present invention relates to a compound semiconductor thin-filmsolar cell having an n-type buffer layer for heterojunction with a lightabsorbing layer and a method of fabricating the same compoundsemiconductor thin-film solar cell.

FIG. 1 shows a basic structure of a thin-film solar cell produced from ageneral compound semiconductor, which comprises a SLG (soda lime glass)substrate 1 on which a back molybdenum (Mo) electrode layer (positiveelectrode) 2, a p-type light absorbing layer 5, an n-type heterojunctionbuffer layer 6 and a transparent electrode layer (negative electrode) 7are subsequently formed in the described order.

In the thin-film compound semiconductor solar cell, the light absorbinglayer 4 is made in the form of a CIGS (Copper-Indium-Gallium-Selenium)thin film made of Cu (In+Ga) Se2 of I-III-V12 group based on Cu, (In,Ga), Se, which possesses high power conversion efficiency exceeding 18%.

U.S. Pat. No. 4,611,091 discloses a method of forming a heterojunctionbuffer layer most suited to a light absorbing layer of CIS by chemicallygrowing a thin film of CdS representing a compound semiconductor ofII-VI group from a solution.

Japanese Laying-Open Patent Publication No. H-8-330614 describes aheterojunction buffer layer of Zn which does not contain harmful metalsuch as cadmium and possesses high power conversion efficiency.

The above-described conventional compound thin film solar cells involvesuch a common problem that a defect easily occur in the junction betweena p-type semiconductor light absorbing layer and an n-type semiconductorbuffer layer because two layers quite differ from each other by theirchemical compositions.

While the light absorbing layer is dipped in the solution, two processesof diffusing Zn element into the light absorbing layer and forming a ZnSfilm concurrently take place, which may easily cause variations in powerconversion efficiency of the product from the crystallinity and surfaceconditions of the light absorbing layer.

To obtain a heterojunction suitable to the light absorbing layer of CIGSthin film, a buffer layer of InS (InS, InO, InOH) is formed by a CBDmethod that has been developed to attain the uniformity of compositionand reproducibility of the product (see reference “Solar EnergyMaterials & Solar Cells” 69, 2001, pp. 131-137).

However, the formation of a buffer layer of InS group by the ChemicalBath Deposition (CBD) method for forming a heterojunction suited to thelight absorbing layer of CIGS thin film still involves a problem thatthe buffer layer of InS has a small band gap and generally is hard topass light of short wavelengths. Namely, it cannot result in high Jsc.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a methodof fabricating a solar cell having an n-type buffer layer providing aheterojunction with a light absorbing layer formed on a back electrode,wherein the buffer layer is formed by CBD (Chemical Bath Deposition)process using a aqueous solution for dipping the light absorbing layerin such a way that particles can be deposited on the light absorbinglayer to form a buffer layer of InS, which has a grain structureimproved to pass even light of short wavelengths. The CBD processaccording to the present invention specifically increases sizes ofparticles to be deposited by increasing temperature of the aqueoussolution.

Another object of the present invention is to provide a method offabricating a solar cell having an n-type buffer layer formed forproviding a heterojunction with a light absorbing layer formed on a backelectrode, wherein the buffer layer is formed by CBD of particles ofn-type semiconductor material. The CBD process according to the presentinvention provides a buffer layer of n-type semiconductor material,which is featured by gradually or step-by-step increased sizes ofdeposited particles in the outward direction from the light absorbinglayer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional illustration of a basic structure of a solar cellof general compound semiconductors.

FIG. 2 illustrates a process of forming a back electrode and a lightabsorbing layer on a substrate of soda lime glass (SLG).

FIG. 3 illustrates a process of forming a buffer layer and a transparentelectrode layer on a light absorbing layer.

FIG. 4 is a graph showing temperature characteristics of aqueoussolution when forming a buffer layer by the CBD method according to thepresent invention.

FIG. 5 is a sectional illustration of forming by CBD a buffer layer ofInS particles on a light absorbing layer.

PREFERRED EMBODIMENT OF THE INVENTION

In FIGS. 2 and 3, there is shown a process of fabricating a compoundsemiconductor thin-film solar cell.

As shown in FIG. 2, a molybdenum (Mo) electrode layer 2 serving as aback electrode is first formed by sputtering on a SLG (soda lime glass)substrate 1. Next, an indium (In) layer 32 is formed on the backelectrode layer 2 by the first sputtering process SPT-1 using a singleIn target T1 and a copper-gallium (Cu—Ga) alloy layer 31 is formedthereon by the second sputtering process SPT-2 using a Cu—Ga alloytarget T2 to form a laminated metal precursor 3 composed of the In layer32 and the Cu—Ga alloy layer 31. The precursor 3 is then treated by heat(by the heat treatment process HEAT) in the atmosphere of selenium (Se)to form a thin film light absorbing layer 5 of CIGS.

As described above, a laminated precursor 3 is fabricated by formingfirst an In layer 32 and then a Cu—Ga alloy layer 31 on a Mo electrodelayer 2, thereby preventing the formation of an alloy of elementsdiffused in solid phase at a boundary between the precursor 3 and the Moelectrode layer 2. This can also facilitate In component to sufficientlydiffuse in the precursor on the side of the Mo electrode layer 2 in theprocess of selenizing the laminated precursor 3 by heating in theselenium atmosphere, simultaneously preventing slowly diffusing elementsGa from segregating at the boundary of the Mo electrode layer 2 andforming thereat a different alloy layer of Cu—Ga—Se which is inferior inits crystal structure. The CIGS light absorbing layer 5 thus fabricatedcan possess high quality P-type semiconductor structure featured by thehomogeneous crystal structure of Cu (ln+Ga) Se2. The light absorbinglayer 5 can be featured by high performance and high strength ofadhesion between the Mo electrode layer 2 and the light absorbing layer5 and is free from the formation of a strange layer (Cu—Ga—Se layer)having an inferior crystal structure and possessing conductivity.Consequently, a solar cell fabricated based on the thus fabricated lightabsorbing layer can possess high strength and is free from leakagefrom/to other cells when it is used in practice.

On the p-type light absorbing layer 5, as shown in FIG. 3, there is thenformed a n-type buffer layer 6 for providing a heterojunction with thep-type layer 5 and a transparent electrode layer 7 of ZnS is furtherformed by sputtering on the buffer layer 6, as shown in FIG. 3.

According to the present invention, the buffer layer 6 of InS is formedby wet chemical bath deposition (CBD) using an aqueous solution ofindium chloride and thioacetamide. In practice, the aqueous solution isprepared as a 1:1 mixture of two solutions: 0.01M/1 ofindium-3-chloride-4-hydrate (InC113.4H2O) and 0.30M/1 of thioacetamide(CH3CSNH2).

The buffer layer 6 is formed by CBD using the above-prepared aqueoussolution according to the following process shown in FIG. 4.

In the first step, the surface of the light absorbing layer 5 is dippedin the aqueous solution at a room temperature T1 (° C.) for a presettime t1 (5-10 minutes) while stirring the solution. Stirring of thesolution is continued until the buffer layer 6 is completely formed.

In the second step, the temperature of the solution is increased to apreset value T2 (about 60° C.) for a preset time t2 (about 10 minutes)while the surface of the light absorbing layer is kept as dipped in thesolution.

In the third step, the dipping of the light absorbing layer in thesolution being kept at the preset temperature T2 is continued furtherfor a preset time t3 (about 40 minutes) after the solution reached thetemperature T2 at the end of the second step. The buffer layer 6 formedon the light absorbing layer is washed with an overflow of pure water.

The above-described process provides an In-layer of fine particlesdeposited by the first step, an In-layer of larger (than those depositedby the first step) particles deposited by the second step and anIn-layer of further larger (than those deposited by the second step)particles deposited by the third step.

This is explained as follows:

The aqueous solution is an almost transparent solution which slightlyassumes a yellow color of thioacetamide at a room temperature.

The process of forming the buffer layer 6 on the light absorbing layerby depositing particles of In with growth of colloids in the aqueoussolution is as follows:

The solution is emulsified as its temperature rises. With progress ofchemical reaction, the emulsion gradually changes its color from deepwhite to yellow. Since InS is an orange solid in itself, the emulsionrepresents that InS-crystals are growing in the solution. The change ofcolor of the solution from white to yellow can be considered to indicatethat InS particles in the solution are further growing to have largersizes.

In other words, the white colloid has particles which are smaller insize than the wavelength of yellow light. Yellow colloid hasIn-particles which are grown large enough to assume its orange color byreflecting orange color light.

The speed of changing color of the solution from white to yellow relatesto a ratio of concentration of indium chloride to concentration ofthioacetamide in the aqueous solution. The color changing speed is aptto increase as the relative concentration of thioacetamide in thesolution is lower.

It has been found that the buffer layer can grow on the light absorbinglayer 5 dipped in the aqueous solution during the time of heating thesolution from the room temperature to the specified temperature but thebuffer layer does not grow in the emulsion having reached the specifiedtemperature.

Accordingly, the process of steps 1 to 3 cause particles of InS todeposit from the solution onto the surface of the light absorbing layerby stepwise increasing sizes of deposits. This can create a buffer layer6 having a structure featured by continuous distribution of Insparticles with stepwise increased sizes. As the result of this, as shownin FIG. 5, the buffer layer 6 tightly adheres to a rough surface of thelight absorbing layer 5 with an improved coverage.

The reason why the layer cannot grow in the emulsion is considered to bethat large particles can have a small contact surface with the lightabsorbing layer 5. It is also considered that the deposit of smallparticles serves as adhesive to grow the buffer layer 6.

The use of a layer of In-particles as the buffer layer 6 offers thefollowing advantage:

-   -   a) Expanding a bandgap by the effect of particle sizes;    -   b) Expanding a bandgap by the effect of particle surfaces;    -   c) Improvement of durability against a plasma damage; and    -   d) Reduction of shunt path by the high resistance of the layer.

It has been considered that the use of the InS-layer in a solar cell isdisadvantageous since InS has a small bandgap and hard to pass light ofshort wavelengths.

The above-mentioned disadvantage can be overcome by making the layercomposed of fine particles of InS. According to the present invention,the quality of the buffer layer is changed by regulating pH of theaqueous solution in steps 1 to 3. In practice, the aqueous solution isused at PH of 1 to 3.5 and 3.5 to 12.0 in steps 1 and 2, respectively,while the aqueous solution is used at pH of 3.5 to 12.0 in step 3. Thus,the lower side deposition of the buffer layer 6 is rich in InS byregulating the pH of the aqueous solution to a acidic value of a pHscale while the upper side deposition is rich in InOH.InO by regulatingthe pH of the solution to a alkaline value.

When pH of the solution of indium chloride and thioacetamide isregulated to about 1-3.5, then the following chemical reaction takesplace:2InC13+3CH3CSNH2+6H2O→In2S3+3CH3CO2+3NH4+6HC1

When pH of the aqueous solution containing trivalent ions is regulatedto 3.4-12, then the following chemical reaction takes place:InC13+3H2O←→In (OH) 3+3HC1

By regulating the pH value of the aqueous solution using theabove-described chemical reactions, the quality of the buffer layer 6can be changed.

Consequently, it becomes possible to achieve the following optimuminterfacial condition between the light absorbing layer 5 and the bufferlayer 6 as well as the buffer layer 6 and the transparent electrode 7.

An In2S3-rich layer obtained by regulating the pH of the solution to avalue of the acidic side can effectively cover the top surface of thelight absorbing layer 5 achieving the junction best suited to the layer5.

To prevent a plasma damage from reaching the junction surface with thebuffer layer 6 in the process of forming the transparent electrode 7, itis necessary to increase the thickness of the buffer layer 6. However,it is disadvantageous to increase the thickness of the InS-rich layerbecause InS has a small bandgap and may deteriorate the opticaltransmittance of the layer. On the contrary, the layer rich inln(OH)3.In203, which is obtained by regulating the pH of the solution tothe alkaline-side value, has a lager bandgap and can be used as atransparent conducting layer. Therefore, it is desirable to form athicker layer rich in In (OH) 3.In203, which can be free from theaffection of plasma damage in the process of forming a transparentelectrode 7 and can attain the suitable conformity to the transparentelectrode 7 and can attain the suitable conformity to the transparentelectrode 7 without decreasing the optical transmittance of the bufferlayer.

INDUSTRIAL APPLICABILITY

As is apparent from the foregoing, according to the present invention,it is possible to provide a method of fabricating a thin-film compoundsolar cell having an n-type buffer layer formed therein for providing ahetero-junction with a p-type compound semiconductor light absorbinglayer formed on a back electrode, wherein the buffer layer is formed byapplying a chemical bath deposition (CBD) process using an aqueoussolution for dipping the light absorbing layer to deposit particles onthe surface thereof. In this process, the temperature of the solution iscontrolled from low to high to increase sizes of the particles to bedeposited on the light absorbing layer so as to form the buffer layerwhich possesses a high optical transmittance, tight adherence to thelight absorbing layer and conformity with the transparent electrodeformed thereon though it is made of InS material generally possessing asmall bandgap and hard to pass light of short wavelengths.

According to the present invention, it is also possible to provide amethod of fabricating a thin-film compound solar cell having an n-typebuffer layer formed therein for providing a hetero-junction with ap-type compound semiconductor light absorbing layer formed on a backelectrode, wherein the buffer layer is formed by applying a chemicalbath deposition (CBD) process using an aqueous solution for dipping thelight absorbing layer to deposit particles on the surface thereof. Theprocess provides a buffer layer of n-type semiconductor material, whichis featured by gradually or step-by-step increased sizes of depositedparticles in the outward direction from the light absorbing layer. Thebuffer layer thus formed possesses a high optical transmittance, tightadherence to the light absorbing layer and conformity with thetransparent electrode formed thereon though it is made of InS materialgenerally possessing a small bandgap and hard to pass light of shortwavelengths.

1. (canceled)
 2. A method of fabricating a thin-film compound solar cellhaving an n-type buffer layer formed therein for providing aheterojunction with a p-type semiconductor light absorbing layer formedon a back electrode, wherein the buffer layer is formed on the lightabsorbing layer by chemical bath deposition (CBD) process using anaqueous solution for dipping therein a surface of the light absorbinglayer, wherein the CBD process comprises a first step of holding thesolution with the light absorbing layer surface dipped therein at afirst preset temperature for a first preset time, a second step ofheating the solution for a second preset time to a second temperaturehigher than the first temperature and a third step of holding thesolution at the second temperature for a third preset time.
 3. A methodof fabricating a thin-film compound solar cell as defined in claim 2,wherein the aqueous solution is stirred all for the first, second andthird steps.
 4. A method of fabricating a thin-film compound solar cellhaving an n-type buffer layer formed therein for providing aheterojunction with a p-type semiconductor light absorbing layer formedon a back electrode, wherein the buffer layer is formed on the lightabsorbing layer by chemical bath deposition (CBD) process using anaqueous solution for dipping therein a surface of the light absorbinglayer, wherein, in the CBD process of forming the buffer layer on thelight absorbing layer whose surface is dipped in an aqueous solution fordepositing particles thereon, pH of the solution is changed from a lowlevel to a high level to cause the buffer layer to have differentquality of deposit layers therein.
 5. (canceled)
 6. A method offabricating a thin-film compound solar cell as defined in claim 2,wherein a pH value of the aqueous solution is regulated to a highervalue in the third step.
 7. (canceled)
 8. A thin-film compound solarcell having an n-type buffer layer formed for providing a heterojunctionwith a p-type semiconductor light absorbing layer formed on a backelectrode, wherein the buffer layer is formed of layered deposits ofparticles of n-type semiconductor material and the layered deposits aredifferent from each other by grain sizes gradually or stepwiseincreasing in the deposits in a direction outward from the lightabsorbing layer.
 9. (canceled)
 10. A thin-film compound solar cellhaving an n-type buffer layer formed therein for providingheterojunction with a p-type semiconductor light absorbing layer formedon a back electrode, wherein the buffer layer is formed of layereddeposits of particles of n-type semiconductor material and the layereddeposits are different from each other by pH-values being smaller inlower side deposit and larger in upper side deposit in a profile of thebuffer layer.